Optical fiber transmission systems predominantly use predetermined “windows” (i.e. bands, channels) of the optical spectrum through which the transmission of the signals along the fibers takes place with a minimum attenuation. Signals or communication channels, each with its own precisely defined wavelength, as produced by a relevant laser generator, included in one of these privileged windows or bands modulable in intensity (commonly in digital or analog mode), may be transmitted along an optical fiber with extremely low losses. The simultaneous transmission of various communication channels belonging to a certain band, or window, or channel on a same fiber, is made possible by operating in Wavelength Division Multiplexing (WDM).
Mach-Zehnder interleavers are well-known devices suitable to realize the insertion or extraction of an optical signal or of a certain optical channel of a certain wavelength on an optical fiber carrying another optical signal or optical channel. With reference to the diagram in FIG. 1, a Mach-Zehnder interleaver may be schematically represented as composed of a first coupler (left side), whose structure is essentially that of two coupled optical paths (for example two waveguides), of an intermediate phase shifting stage suitable to determine a certain phase difference Δφ by means of a difference n·ΔL of the optical path (where n is the refractive index and ΔL is the geometrical path difference) on the two branches of the device, and of a second coupler (right side).
The characteristic attenuation curve of a classic Mach-Zehnder interleaver is substantially of periodic type, as shown by way of example in FIG. 2, and is characterized by relatively selective peaks which are exploited to inject a certain frequency (wavelength centered to one of these peaks) in fiber and/or to extract it. Therefore a certain transmission system with optical fibers, operating with carrying signals with a wavelength included in the pass-band of a first channel or window, whose central or main wavelength is λ1, may also support transmissions made in a second channel or window, whose central or main wavelength is λ2. This result is obtained by determining the optical path difference ΔL so as the cross power transfer ratio PLH-RL from a first port (left-high, LH) to a third port (right-low, RL) is maximum at the wavelength λ2 and practically null at the wavelength λ1, and the parallel power transfer ratio PLH-RH from the first port (left-high, LH) to the second port (right-high, RH) is maximum at the wavelength λ1 and practically null at the wavelength λ2.
As shown in FIG. 1, Mach-Zehnder (MZ) interleavers can be used in both directions, either from left to right or vice versa, to separate (demultiplexer) two superposed optical signals applied at a same port or to superpose (multiplexer) two separated optical signals applied at two distinct ports, respectively. In both cases, in ideal functioning conditions the superposed (multiplexed) optical signals are available at the first port only and there is a fourth port (the left-low port LL in FIG. 1) at which there is not any optical signal in the shown configuration.
A plurality of MZ-interleavers MZ-1, MZ-2, MZ-3 may be coupled together as shown in FIG. 3 to implement an optical multiplexer/demultiplexer. An exemplary graphic representation of the four power transfer ratios P of the optical multiplexer/demultiplexer of FIG. 3 in function of the wavelength of input optical signals is depicted in FIG. 4.
In order to make MZ-interleavers work correctly, the optical path difference n·ΔL of each interleaver is controlled using actuators, depicted as gray rectangles, for compensating eventual temperature fluctuations, tolerances of fabrication etc. To this end, as shown in FIG. 5, the fourth port of each MZ-interleaver may be coupled to a photo-detector PD. If the optical path difference n·ΔL does not have the correct value to maximize the power transfer ratio at the normally used ports, then a nonnull optical signal is received at the fourth port revealing that the MZ-interleaver is not working in ideal conditions. The optical signals eventually sensed by the photo-detectors PD are converted into electric error signals provided as input to respective control blocks CONTROL that command the actuators (represented with gray rectangles) for adjusting the optical path differences of the MZ-interleavers.
Unfortunately, this control scheme cannot be implemented when the cascade of MZ-interleavers is used as an optical demultiplexer, as shown in FIG. 6, because no signal will ever be available on the normally unused ports.